Incubator humidity guide
Humidity is the setting people worry about most and get wrong most often. The good news: you don't have to hit a magic number. You have to let the eggs lose the right amount of water.
The rule: eggs must lose about 13 % of their weight
An egg loses water through its shell for the whole incubation. A chicken egg should lose roughly 11–14 % of its starting weight by lockdown, about 13 % on average. That loss is what creates the air cell the chick breathes from before it hatches.
- Too humid (too little loss): small air cell, large and soft chicks that struggle to turn and pip, some drown in the shell, and mushy, smelly navels.
- Too dry (too much loss): large air cell, small weak chicks, membranes that dry onto the chick ("shrink-wrapped"), more so if the lid is opened during the hatch.
Humidity is just the tool to steer that weight loss. Shell thickness, egg age and your climate all change how much humidity a particular batch needs.
Starting values by species
| Bird | Day 0 to lockdown | Lockdown to hatch |
|---|---|---|
| Chicken | 45–50 % | 65–70 % |
| Duck | 50–55 % | 65–75 % |
| Muscovy duck | 50–55 % | 65–75 % |
| Goose | 50–55 % | 70–75 % |
| Turkey | 50–55 % | 65–70 % |
| Coturnix quail | 45–50 % | 65–70 % |
| Bobwhite quail | 45–50 % | 65–70 % |
| Guinea fowl | 45–55 % | 65–70 % |
| Pheasant | 45–50 % | 65–70 % |
| Partridge (chukar) | 45–50 % | 65–70 % |
| Peafowl | 50–55 % | 65–70 % |
Relative humidity in a forced-air incubator. Full chart with temperature and days: incubation chart.
Check it: egg weight-loss calculator
Weigh a few marked eggs (or the whole tray) on a kitchen scale on day 0, then again on a later day. Enter both weights and the calculator compares the loss with the 13 % target.
Check it without a scale: the air cell
Candle the eggs on the usual days and draw the air cell outline in pencil. A healthy chicken egg's air cell is small on day 7, about a quarter of the egg on day 14 and around a third at lockdown, with the edge dipping to one side just before the hatch. If the cells are smaller than that, lower the humidity; if larger, raise it. See the candling guide for pictures.
The dry hatch
A "dry hatch" means running the incubator with no added water (or very little) until lockdown, often 25–40 % relative humidity depending on the room. It works well for many keepers in humid climates or with thick-shelled eggs, because the room's own humidity is already high. In a dry climate or a heated room in winter, it can dry the eggs out. Treat it like any other setting: watch the air cells or the weight loss and adjust. Humidity still goes up at lockdown.
Calibrate your hygrometer (salt test)
Cheap hygrometers are often 5–10 % out. To check yours for free:
- Put a tablespoon of table salt in a bottle cap and add a few drops of water so it becomes a damp paste, not a liquid.
- Seal it in a zip bag with the hygrometer, away from sun and heat, for 8–12 hours.
- Saturated salt holds the air at about 75 %. Note how far off your reading is and correct for it.
Common fixes
- Humidity too low: add water surface area (a wider tray or a sponge), not just more water.
- Humidity too high: remove water, open the vents a little, or move the incubator to a drier room.
- Humidity drops at hatch: fill the channels before lockdown, use warm water, and keep the lid shut.